sandbox: Generalise SPL booting

sandbox: Generalise SPL booting

At present sandbox only supports jumping to a file, to get to the next
U-Boot phase. We want to support other methods, so update the code to
use an enum for the method. Also use the

Use board_boot_order() to set the order, so we can add more options.
Also add the MMC methods into the BOOT_DEVICE enum so that booting
from MMC can be supported.

Signed-off-by: Simon Glass <sjg@chromium.org>